The invention relates to a device for treating one surface of a web by means of at least one roller-shaped working member, especially a roller squeegee, which is attracted by a magnetic element situated on the other side of the web and consisting substantially of a series of electromagnets arranged one after another in the longitudinal direction of the magnetic element. The present invention is particularly suitable for use in a screen stencil printing device.
The particular operating conditions of such devices require that steel rollers of different diameter be employed as the roller-shaped working members, e.g. roller squeegees. For example, if in a screen stencil printing device only a relatively small amount of material per unit of surface is to be applied to the surface of the web and a good sharpness of outline, e.g. of a printed pattern, is required, a small diameter roller squeegee may be satisfactorily employed. On the other hand, a larger diameter roller squeegee is necessary when a greater amount of coating material is to be applied to the web.
In known magnetic elements the coils are connected so as to produce an alternating north-south polarity of the magnet cores. Together with the two adjacent poles, each magnetic pole generates a magnetic field of force which has a strong lateral dispersion but the depth of which is not very strong. When the web to be worked is relatively thin and when the steel rollers employed as roller squeegees or other working elements possess a diameter of up to about 15 mm, a magnetic field of the type described is most suitable. On the other hand, if steel rollers of a larger diameter are to be used, or if the distance between the magnetic poles and the steel roller is relatively greater due to the thickness of the material, the attraction of the magnetic field is not sufficient for holding and maintaining the steel roller in the desired position.